1. Field of the Invention
This invention relates to mixers in general and more particularly to a dual double balanced mixer that minimizes inter-modulation products and has a high third order intercept point (IP3).
2. Description of the Prior Art
A mixer circuit converts an RF signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates intermodulation products due to the non-linearity response. Third order intermodulation products are close in frequency to the fundamental IF frequencies and therefore are difficult to remove by filtering. Third-order intermodulation distortion is a measure of the third-order products generated by a second input signal arriving at the input of a mixer along with the desired signal.
One technique to measure the suppression capability of a mixer is the xe2x80x9cthird-order interceptxe2x80x9d approach. The third-order intercept point is a theoretical point on the RF input versus IF output curve where the desired output signal and third-order products become equal in amplitude as RF input is raised. The high end of the dynamic range of the mixer is defined as the maximum received signal power at which the mixer is designed to be used and is designated the 1 dB compression point or the input signal power level at which the power level of a third order product equals the power level of a fundamental IF signal is called the third order intercept point (IP3). A mixer with a higher IP3 value will have better performance. A mixer is usually specified in terms of input IP3. Output third order intercept point is the difference between input IP3 and conversion loss. Higher conversion losses result in lower output IP3.
Conversion loss is a measure of the efficiency of the mixer in providing frequency translation between the input RF signal and the output IF signal. Conversion loss of a mixer is equal to the ratio of the IF output to the RF input level.
Mixers are typically designed with one of three topologies: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the RF signal and the local oscillator LO voltages from the output and thus allow overlap of the RF and IF frequency bandwidths. Several prior art mixer circuits are well known. One mixer design uses a schottky diode quad or ring circuit that uses four diodes with all of the diodes pointed in the same direction. Another mixer circuit is called a star circuit, which uses two diodes pointing toward the central node and two diodes pointing away from the central node. Unfortunately, mixers with schottky diodes are limited to an IP3 of 25 to 30 dBm. Schottky diode mixers approaching 30 dBm are difficult to tune and are expensive. Diode mixers also require large LO signal levels to obtain a high IP3 which is not practical in many systems.
Another type of mixer uses field effect transistors (FET) as the mixing element instead of a schottky diode. Mixers fabricated using FET""s can achieve a higher value of IP3 greater than 30. Unfortunately, mixers using FET""s have several other disadvantages such as higher conversion losses of approximately 9 dB, higher noise figure (conversion losses at +1 dB) and the need for DC current.
While double balanced diode and FET mixers have been used, none of these will give a high IP3 in a double balanced mixer configuration without substantial loss. A current unmet need exists for an improved double balanced mixer with that has a high third order intercept point, low conversion loss, low noise figure and does not require DC power.
It is a feature of the invention to provide a double balanced mixer for mixing an RF input signal with a local oscillator signal to provide at an output an intermediate frequency signal with a high third order intercept point.
Another feature of the invention is to provide a double balanced mixer that includes a first and second local oscillator balun that is operable to receive a local oscillator signal. A first and second RF balun is operable to receive a RF signal. A first mixer has a first input port coupled to the first local oscillator balun, a second input port coupled to the first RF balun, and an output port that provides an intermediate frequency signal. A second mixer is coupled in parallel with the first mixer. The second mixer has a first input port that is coupled to the second local oscillator balun, a second input port that is coupled to the second RF balun, and an output port that provides an intermediate frequency signal. A first intermediate frequency balun is coupled to the output port of the first mixer and a second intermediate frequency balun is coupled to the output port of the second mixer. The parallel connected first and second mixers provide a high third order intercept point. The first and second mixers are each field effect transistor quads.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, neither is it intended to be limiting as to the scope of the invention in any way.